Chapter I. On the Properties of Air.

Indeed, if the outside air were annihilated, or if its elasticity were much less than that of the air inside, then from now on we shall treat the weight of the air as an expression of its elastic force, or its elasticity as its weight. When one is in a place higher or lower than sea level, one can always judge the approximate elastic force of the air in that location by the height of the barometer that has been carried there.

The elastic force of the air is the cause of the difficulty experienced in separating two polished bodies. Fig. 1.

796. Once one understands the weight and elasticity of the air, it is easy to explain several effects of nature that the Ancients attributed to the original: "l'horreur du vuide" horror of a vacuum: for example, experience shows that if one has two highly polished bodies, such as two mirror plates, applied one against the other and touching across every part of their surfaces, one finds great difficulty in separating them. This is because, there being no air between them whose elasticity can balance the column of air pressing down with its weight on the two bodies, one must overcome the weight of the entire column of air, which would have for its base the surface touching the other.

The reason why one cannot open a bellows whose openings are all plugged without great effort. Fig. 15 & 16.

Similarly, if one has a closed bellows whose nozzle and valve are tightly plugged, and one attaches one of the wings against a vertical or horizontal surface, one cannot open the bellows—that is, move the other wing away from the first—without overcoming the resistance of a large part of the column of air, which would have for its base one of the wings of the bellows. Since very little air remains in the original: "l'ame," literally "the soul," referring here to the internal cavity or heart of the bellows. interior when the bellows is expanded, the air that wishes to occupy that space cannot re-enter the internal capacity; it resists with a force that one would hardly believe if experience did not confirm it.

That the weight of the air is the cause of the property of the siphon. Fig. 14.

797. To explain how the weight of air moves water from one vessel into another with the help of a siphon A tube used to convey liquid upwards from a reservoir and then down to a lower level of its own accord., one must understand that vessel D, where the water is located, must be slightly higher than the other vessel E, where it is to go; and that the siphon A, which is nothing more than a tube of copper or tin-plate, has one branch B shorter than the other branch C. To use it, one fills the siphon with water to expel the air; then one plugs the two holes very exactly, turns the siphon over, placing the shorter branch to soak in vessel D, and unplugs it within the water itself; one also unplugs the other branch C. Then, one sees all the water from one vessel pass into the other, which occurs because there is a greater height of water in branch C than in the other branch B. For at first, the air acts on both sides to make the water rise higher than the summit A of the siphon, but it is pushed back with more force by the water in branch C than by that in branch B.